In a steam turbine power plant, a boiler heats feedwater into steam which is then introduced into the steam turbine to turn a bladed rotor and thus produce a power output. The power output could be used to drive another mechanical device or it could be used to drive an electrical generator to produce electrical energy. The steam within the steam turbine may be extracted for preheating feedwater and it may also be used for providing process steam to a steam consuming process. The remainder of the steam is exhausted to a steam Condenser where it becomes feedwater again to be recirculated to the boiler. Makeup water may also be added to the feedwater as the feedwater level is diminished.
Prior to being introduced into the boiler, the feedwater may be preheated by a number of devices. One such device, a generic feedwater heater, provides non-contact heating of the turbine feedwater by passing a plurality of water filled tubes through a steam filled container whereby heat from the steam is used to heat the feedwater. Another water heating device is a deaerator which utilizes steam extracted from a steam turbine to preheat feedwater which is sprayed into the steam chamber of the deaerator. The deaerator is a direct contact feedwater heater which heats feedwater and removes non-condensible gases. The deaerator is pressurized by steam extracted from the steam turbine and it is important to avoid rapid depressurization in the deaerator in order to control the volume swell of water held within the deaerator while the water is being heated. If rapid depressurization happens, flooding could occur causing water slugs in the steam inlet line or damaging deaerator trays which may necessitate plant shutdown.
The deaerator contains a hot well from which feedwater is pumped to the boiler by a boiler feed pump. The rate of pressure decay in the hot well must not exceed a value which would cause flooding and there is a minimum pressure which must maintained to avoid cavitation at the boiler feed pump suction. This minimum pressure is sometimes referred to as the net positive suction head. The net positive suction head (NPSH) is the energy required to fill a pump on the suction side and overcome friction and flow losses from the suction connection to that point in the pump at which more energy is added.
Normally, pressure is maintained in the deaerator by means of uncontrolled extraction steam from the steam turbine. However, the pressure at which this steam is supplied is subject to fluctuations that occur upon changes in process steam demand or power plant demand. These fluctuations are directly reflected in deaeration pressure and must be controlled to avoid the previously mentioned problems.
One prior art solution to the problem is to assure a second source of steam pressure to the deaerator which is utilized when steam pressure falls below a predetermined minimum pressure. Such a source of steam could be a process steam line which is tapped through a control valve to add steam whenever the steam pressure of the deaerator falls below the predetermined minimum pressure. However, the rapid decrease in the supply of steam to the deaerator and consequent rapid depressurization of the deaerator could result in severe misoperation before the minimum pressure point and the corrective action that would trigger from a second steam source.
U.S. Pat. No. 4,345,438 to Labbe et al and assigned to the assignee of the present invention is relevant in that it provides a controller for avoiding deaerator flooding due to rapid depressurization caused by operating transients. The solution in that patent is to reduce the inflow of feedwater into the deaerator in response to a signal indicative of turbine load.
It is an object of the present invention to provide a system for avoiding flooding in a deaerator by controlling the pressure in the deaerator.
It is another object of the invention to provide a system for avoiding flooding in a deaerator even before a critical low pressure in the deaerator is reached.
It is another object of the invention to provide a system which will assure adequate pressure for avoiding upset in the boiler feedwater pump.
The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof may best be understood with reference to the following description and drawings.